We use EPR spectroscopy to probe the environment of the heme moity in recombinant adult hemoglobin produced in E. coli. UV and visible spectroscopy sugge st that some portion of the protein subunits may possess a sulfur-derivated heme prosthetic group described elsewhere as sulf-hemoglobin. This small (5%) conta minating amount of sulf-heme may explain the reduced cooperativity of hemoglobin produced by bacterial expression. EPR analysis shows significant differences i n the low spin hydroxy bound species in the g=2 area of the spectra between native and recombinant.